Minimum Shift Keying (MSK) is a communications protocol that is commonly used to communicate data in communications networks. MSK uses continuous phase modulation techniques to communicate the required data. One aspect of processing received signals involves acquisition and estimation of the signal in time, phase, frequency, and signal quality. The acquisition is often performed using correlation techniques. The correlation techniques attempt to compare any incoming signals with a synchronization pattern known to both a transmitter and a receiver. The transmitter inserts the known synchronization pattern into the transmitted signal. The receiver uses this pattern to perform the acquisition and estimation of the incoming signal.
Often, a signal is represented using in-phase and quadrature components of the received signal to assist in processing. FIG. 2 shows how the quadrature or phase angle component 30 of an MSK signal changes in response to an on/off bit pattern 32. It can be seen that a change in the bit pattern results in an immediate reversal in direction of the phase rotation (frequency). One drawback to using an MSK strategy is that frequency changes, such as at 34, provide possible discontinuities that result in wide bandwidth frequency events thus giving up spectral efficiency.
Gaussian Minimum Shift Keying (GMSK) provides at least a partial solution to the frequency discontinuities inherent in MSK techniques. As shown at reference number 38 in FIG. 3, GMSK techniques round off or smooth each phase change such that the phase change does not yield sudden shifts in frequency (dφ/dt). However, because of the rounding off of the signal, GMSK techniques suffer from inherent intersymbol interference. FIG. 4 depicts “eye diagrams” for the in-phase and quadrature components of an ideal MSK signal 40. The best point to sample the signal for correlation purposes is the point 42 at which MSK eye diagram 40 is widest. This is when the phase angle is at 90 degrees and integer multiples thereof, and corresponds to the times when the on/off bit pattern can change from a +1 value to a −1 value. However, the eye diagrams for a the in-phase and quadrature components of a GMSK signal 44, as represented in FIGS. 5A and 5B, show that at phase angles ideal for MSK signals (at point 46), the ideal GMSK signal does not provide a predictable maximum opening or predictable phase. This demonstrates that GMSK signals are inherently susceptible to intersymbol interference at the phase angles of 0, 90, 180, and 270 degrees, said interference being usually in the neighborhood of 2-3 dB. Consequently, when a GMSK signal is correlated at the same phase angles as MSK signals, correlation computations may be adversely affected.
It is therefore an object of the invention to provide a method of correlating signals having in-phase and quadrature components.
It is also an object of the invention to provide a method of correlating such signals while minimizing the effects of intersymbol interference.
A feature of the invention is using knowledge of direction of phase rotation due to two consecutive bits in a known synchronization key to create a correlator synchronization pattern.
An advantage of the invention is the effective elimination of intersymbol interference while simplifying the computation required by a correlator function.